Circadian rhythms have long been known to influence behavioral and biological processes such as physical activity and feeding behavior. The fundamental importance of this system, which works to link physiology with the day/night cycle, is underscored by its presence in every known species and the growing evidence linking alterations in core clock function and diseases such as cancer, diabetes and mental health. Recent studies from our laboratory have shown that mutations of the canonical circadian genes, Clock and Bmal1, dramatically disrupt muscle function. This is the first genetic evidence linking skeletal muscle function to circadian rhythms. Expression profiling studies of skeletal muscle over 48 hours identified that MyoD is a circadian gene. Molecular experiments went on to demonstrate that the MyoD promoter is directly bound and regulated by the core clock factors, CLOCK and BMAL1. The studies outlined in this proposal combine molecular, cell biological and biophysical approaches to understand the mechanisms by which the core clock mechanism is regulated in skeletal muscle and how loss of this rhythm leads to altered MyoD expression and loss of muscle function. The hypotheses for this proposal are 1) Synchronization of core clock gene expression in skeletal muscle requires intact innervation, 2) Muscle specific mutations of Bmal1 or Clock will be sufficient to cause arrhythmic MyoD expression and muscle dysfunction 3) Loss of maximal force capacity/specific tension in muscle of circadian clock- compromised mice is due to decreased actin:myosin interaction resulting from disruption of sarcomeric structure and/or altered stoichiometry of myofilament proteins. The results of the experiments outlined in this proposal have significant implications for maintenance of muscle with an impact on our understanding and treatment of sarcopenia, muscle wasting/cachexia and problems associated with metabolic diseases such as insulin resistance. In addition, our understanding of circadian regulation of skeletal muscle will have likely applications for rehabilitation therapies for spinal cord injury patients and people that are exposed to prolonged periods of bed rest. PUBLIC HEALTH RELEVANCE. The results of the proposed work will determine the role of the biological clock in maintaining normal skeletal muscle structure and function. The findings have significant implications for maintenance of muscle with an impact on our understanding and treatment of sarcopenia, muscle wasting/cachexia and problems associated with metabolic diseases such as insulin resistance. In addition, our understanding of circadian regulation of skeletal muscle will have likely applications for rehabilitation therapies for spinal cord injury patients and people that are exposed to prolonged periods of bed rest.